This invention relates to the production of one-side electroplated steel coiled strip or sheets with the non-electroplated side having good conversion coating characteristics. Hereafter, coiled strip or sheets will be collectively referred to as strip.
The term one-side electroplated strip means a steel strip having a metallic coating electroplated onto one side of the strip while the opposite side of the strip is free from the electroplated metallic coating. The opposite side free from metallic coating hereafter will be referred to as the non-electroplated side to distinguish it from the electroplated side even though the non-electroplated side may have a thin electroplated metallic coating temporarily deposited thereon during the process of this invention.
One-side electroplated strip is extensively used in the automotive industry with the non-electroplated surface becoming a painted exposed surface and the electroplated surface having good corrosion resistant characteristics. Prior to painting, the non-electroplated surface is pretreated to improve paint adhesion. This pretreatment or chemical conversion treatment is usually a phosphate coating.
As is well known, the surface to be electroplated must be cleaned to remove dirt, oil film and the like. This cleaning is accomplished by alkaline cleaning followed by pickling. Although immersion pickling could be used, this invention is limited to electrolytic pickling because the latter is more efficient. Electrolytic pickling includes using one or more pairs of electrodes positioned on each side of a passing strip. These electrodes normally are cathodes. When several pairs of electrodes are used such as in a vertical pickling unit, one or more of the pairs may be anodes. Hydrogen or oxygen gas forms at the strip surface which tends to scrub and more thoroughly clean the steel strip.
After being cleaned, the steel strip is passed through the electroplating unit containing one or more electroplating cells. Current flows between one or more electrodes and the strip surface causing metal to be deposited from the electrolyte onto one side of the strip. A yellow or dark stain caused by oxidation of the unplated surface by the electrolyte may form on the other side of the strip not electroplated. The non-electroplated side may also become etched by the electrolyte. This contamination and/or etching may result in uneven phosphate deposition and large phosphate crystals resulting in poor paint gloss, poor paint adherence and inferior corrosion resistance after painting. Good phosphating characteristics as defined by the automotive industry requires a uniform appearance of the phosphated surface, a dense microstructure i.e. crystal size &lt;25 microns, and a smooth or unetched surface.
There have been various suggestions for removing the stain and preventing etching of the non-electroplated surface. U.S. Pat. No. 4,632,733 discloses wetting the stained surface with a weak acid and a saturated monovalent alcohol. The wetted surface is then abraded using a grit coated brush to remove the stain. U.S. Pat. No. 4,464,232 discloses slightly plating the non-electroplated side while electroplating the other side. After electroplating, electrolysis is applied to the non-electroplated side to remove the thin plating metal. Thus, deposition of corrosion products to the non-electroplated side from the electrolyte is prevented. U.S. Pat. No. 4,609,594 discloses electroplating one side of a steel strip with a plating metal followed by plating the non-electroplated side with a layer of oxides. The oxide plated side is then given a cathodic treatment to remove the oxides thereby enhancing the phosphating properties. U.S. Pat. No. 4,708,779 discloses electroplating one side of a steel strip with zinc. A solution of bifluoride salt is applied to the non-electroplated side of the strip and followed by rinsing with a dilute caustic solution. Phosphatability of the non-electroplated side of the strip is enhanced.
Accordingly, there is a concern about the conversion coating characteristics of the non-electroplated surface of one-side electroplated strip. More particularly, a procedure is needed to prevent staining and etching of the non-electroplated surface by the plating electrolyte while electroplating the other strip surface. We have determined staining and etching of the non-electroplated surface of the strip can be minimized by cathodic pickling the non-electroplated surface immediately prior to electroplating. An anode is positioned in a pickling solution adjacent to the non-electroplated strip surface. Current is passed through the anode and applied to the non-electroplated surface thereby providing a surface which has excellent conversion coating characteristics.